This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Divalent cations play a critical role in the folding and activation of RNA. Unlike DNA, where the canonical structure is well defined, RNAs are found to adopt a wide range of structures. In many cases, divalent cations (Mg2+, Mn2+, and Zn2+) are required for the RNA to adopt a biologically active structure. Recent NMR studies have provided direct evidence for a unique binding site of the metal to the RNA and have suggested that the metal forms an inner-sphere complex with the RNA. EXAFS will be used to test this proposal and to better define the nature of the RNA-metal interaction. In order to define specifically the RNA-metal binding site, thio-modified RNA will be used to label possible backbone phosphates.